The invention relates generally to treatment of emissions in an exhaust path for a combustion system and more specifically to a method and apparatus for reducing NO2 formation in the exhaust path.
FIG. 1 is a schematic illustration of an exemplary gas turbine system 10 including an intake section 112, a compressor section 114 coupled downstream from the intake section 112, a combustor section 116 coupled downstream from the intake section 112, a turbine section 118 coupled downstream from the combustor section 116, and an exhaust section 120. Turbine section 118 is rotatably coupled to compressor section 114 and to a load 122 such as, but not limited to, an electrical generator and a mechanical drive application.
During operation, intake section 112 channels air towards compressor section 114. The compressor section 114 compresses inlet air to higher pressures and temperatures. The compressed air is discharged towards to combustor section 116 wherein it is mixed with fuel and ignited to generate combustion gases that flow to turbine section 118, which drives compressor section 114 and/or load 122. Exhaust gases exit turbine section 118 and flow through exhaust section 120 to ambient atmosphere.
During the combustion of natural gas and liquid fuels, pollutants such as, but not limited to, carbon monoxide (CO), unburned hydrocarbons (UHC), and nitrogen oxides (NOx) emissions may be formed and emitted into an ambient atmosphere. CO and UHC are generally formed during combustion conditions with lower temperatures and/or conditions with an insufficient time to complete a reaction. In contrast, NOx is generally formed under higher temperatures. At least some known pollutant emission sources include devices such as, but not limited to, industrial boilers and furnaces, larger utility boilers and furnaces, reciprocating engines, gas turbine engines, steam generators, and other combustion systems.
Modern air quality regulations mandate continuingly reduced emission levels for power generating plants, while at the same time fuel efficiency requirements continue to increase. Due to stringent emission control standards, it is desirable to control NOx emissions by suppressing the formation of NOx emissions. Nitrous oxides include NO and NO2 where NO2 is known to produce a visible yellow plume from exhaust stacks and further create “acid rain”. However, combustion controls alone may prove inadequate to satisfy these often-conflicting goals, and thus continued the improvement of post-combustion exhaust gas treatment systems is desired.
One technology for the control of oxides of nitrogen that is currently being used commercially at large land-based electrical power generating stations is selective catalytic reduction (SCR). The flue gases from a power station have a net oxidizing effect due to the high proportion of oxygen that is provided to ensure adequate combustion of the hydrocarbon fuel. Thus, the oxides of nitrogen that are present in the flue gas can be reduced to nitrogen and water only with great difficulty. This problem is solved by selective catalytic reduction wherein the flue gas is mixed with anhydrous ammonia and is passed over a suitable reduction catalyst at temperatures between about 150-550 degrees C., and preferably between 300-550 degrees C., prior to being released into the atmosphere. The ammonia is not a natural part of the combustion exhaust stream, but rather, it is injected into the exhaust stream upstream of the catalyst element for the specific purpose of supporting one or more of the following reduction reactions in Equations (1) to (3):4NH3+4NO+O2→4N2+6H2O  (Equation 1) (fast reaction);4NH3+2NO+2NO2→4N2+6H2O  (Equation 2) (fast reaction); and8NH3+6NO2→7N2+12H2O  (Equation 3) (slow reaction: more catalyst surface required).The predominate reaction for NOx removal is Equation (1), assuming the exhaust gas is mostly NO. When the mole ratio of NO to NO2 is greater than 1.0, the reaction of Equation (2) is also fast, reacting equal moles of NOx to NH3. Higher molar ratios of NO2 are reduced through a much slower reaction of Equation (3) requiring a higher space velocity (longer catalytic reactor bed). The third reaction requires one third more ammonia to reduce NO2 than the second reaction, increasing total ammonia consumption. The NOx reduction is primarily dependent on temperature. A given catalyst will generally exhibit optimum performance within a temperature range of plus or minus 50 degrees R (Rankine), where flue gas oxygen concentrations exceed one percent. Below this, the catalyst activity is greatly reduced, thus allowing some unreacted ammonia to slip through. Excessive temperatures may also damage the catalyst. Further, above the optimum temperature range, the ammonia itself will be oxidized to form additional NON according to Equation (4):4NH3+5O2→4NO+6H2O  (Equation 4).
It is also known to combine an SCR process with a catalytic oxidizing process to treat an exhaust gas flow by oxidizing carbon monoxide to carbon dioxide and by oxidizing hydrocarbons to carbon dioxide and water. The oxidizing process is typically located upstream of the ammonia injection location and upstream of the reducing catalyst, because the oxidizing catalyst will also function to oxidize ammonia, which is undesirable as it decreases the amount of ammonia available for reduction of the NOx and because it produces additional NOx compounds.
FIG. 2 provides a simplified exemplary illustration of a combined cycle power plant 200. Air 210 is received in air intake 215 of compressor 220 to provide compressed air for mixing with fuel 225 in combustors 230 to supply hot gases to gas turbine 235 for driving shaft 236 connected to generator 240 for producing electricity output 245. Exhaust gases 250 are discharged into exhaust duct 255, through heat recovery steam generator 260 and out through stack 265 to atmosphere 270. The heat recovery steam generator (HRSG) includes heat exchangers 262 to extract heat from the exhaust gases 250 and emissions treatment equipment 264 for emission controls. The heat extracted from the exhaust gases is used to generate steam 280. The steam 280 is supplied to steam turbine 282 to drive shaft 290 of generator 292 for producing electricity. The steam 280 then passes to condenser 284 where cooling water 286 passing through tube bundles condenses the steam to water 288. The water 288 is then returned to the HRSG for completion of a closed cycle. An operating HRSG may include multiple heat exchangers and evaporators, steam systems and water systems for producing steam at different pressures and temperatures in many different configurations. Similarly, the emissions treatment equipment may include multiple treatment elements within the HRSG and adapted to address different pollutants in different ways.
Arrangement of the above-described catalytic elements may influence the overall performance in treatment of NOx and other emissions being discharged into the atmosphere. Therefore, a need exists to arrange such elements to reduce discharge of NOx and other discharged pollutants.